Our goals are to understand the genetics and organization of rabbit immunoglobulin genes and the mechanisms that regulate their expression during lymphoid cell differentiation. We define structural differences between genetically controlled rabbit Ig allotypes using antisera we prepare and characterize. We conduct breeding experiments with rabbits of defined allotypes and study their cells as they exist in vivo and after culture in vitro. In b4b5 rabbits suppressed for paternal allotype, cells with surface lg of that type appear by 1 year and are found in proportions higher than are expressed in serum. A subpopulation of small pre-B cells (approximately 20 percent) express Kappa light chains and exhibit allelic exclusion. In heterozygotes, kappa allotypes that are inbalanced in expression in adults are expressed in balanced proportions in pre-B and B cells of newborns. Mutant Basilea rabbits produce b9-like light chains detectable in their pre-B cells. Cell lines from rabbit splenocytes have been maintained in culture for more than 6 months, are being subcloned and characterized as B cells, and will be used for regulation, RNA and DNA studies. We have typed F2 and backcross progeny with antisera to bas kappa allotype and demonstrated that this trait behaves as an allele (or pseudoallele) of b4, b5 and b6 allotypes.